Various pieces of outdoor power equipment, such as lawn mowers, snowblowers and the like, are self-propelled by small mechanical transmissions. These transmissions usually have different speeds which the operator can select using a control lever. The transmission is usually contained inside the housing of the device while the control lever is easily accessible to the operator. For example, in a lawn mower, the transmission is mounted beneath the mower deck while the control lever is up on the control panel of the lawn mower handle assembly.
U.S. Pat. No. 3,812,735 to Von Kaler discloses a typical transmission of this type known as a draw key transmission. Such a transmission includes a shaft having a plurality of gears of different gear ratios rotatably journalled thereon. A slidable key contained in a keyway in the shaft is used to couple one of the gears to the shaft to provide a first output speed for the transmission. The key can be selectively moved in the keyway to be coupled to any of the other gears to change the transmission output speed in accordance with the gear ratio of the selected gear.
Transmissions of this type desirably have a detent means for indicating to the operator when the key is coupled to each of the gears. The detent means lets the operator feel when the key "clicks into place" in each gear. This has sometimes been done using a detent structure remotely located from the transmission. For example, in certain prior art lawn mowers manufactured by The Toro Company, the assignee of this invention, the detent structure is located within the control lever assembly mounted on the control panel. Movement of the control lever is transmitted to the shift key through an elongated cable.
The length and the adjustment of the control lever cable is difficult to control precisely. Thus, it often happens that the detent positions provided by such a "remotely located" detent structure become misaligned with the actual positions of the key in the keyway. In other words, the operator moves the control lever until reaching a detent position, but the key is not yet engaged in any gear, i.e. it is between gears. Thus, the operator has to keep moving the control lever forwardly from the detent position "hunting for" the spot at which the gear will become engaged. This is most often done by moving the control lever and simultaneously engaging the transmission until the key pops into place in the gear allowing the mower to begin moving. However, such a procedure is annoying and confusing to the operator and is obviously undesirable.
In some cases, the detent structure is provided as part of the transmission itself, rather than being remotely located from the transmission as described above. For example, in the Von Kaler patent referred to previously, a rotary shift shaft is connected to the shift key through a fork and collar arrangement for sliding the key in the keyway by rotating the shift shaft. In addition, a detent plate having a plurality of spaced detents in an arcuate edge thereof is carried on the shift shaft. A spring biased ball located on a wall of the transmission housing engages the detents to define the different detent positions corresponding to engagement of the respective change speed gears.
While the detent positions provided by the ball/plate structure of Von Kaler theoretically are more aligned with the gear engaging positions of the key, it is still not a direct measurement of key location. It depends on the accurate location of the detent plate on the shift shaft during manufacture of the transmission. It also depends on the detent plate maintaining the same relative location on the shift shaft during operation of the transmission over extended periods of time. Any relative rotation of the shift plate on the shift shaft, or improper placement of the shift plate in the first place, will cause the same kind of problem referred to above, namely the detent positions will not precisely correspond to gear engagement positions of the key.
In addition, many of the detent structures of the prior art include numerous parts added to the transmission or the device in which the transmission is used. Referring again to the Von Kaler structure, the detent means includes the detent plate, the ball, the biasing spring for the ball, and the structure for housing the ball and spring. These additional parts add additional expense to the transmission, which is also undesirable.